Recently, ionomer resin has been widely used for cover material of golf balls. This is because the ionomer resin is superior in rebound characteristics, durability, processability and the like. However, since the ionomer resin has high rigidity and hardness, there are problems that in the resulting golf ball, shot feel is hard and poor; and spin performance is not sufficiently obtained, which degrades controllability, when using ionomer resin alone as the cover material.
In order to solve the problems, it is attempted to soften the ionomer resin by various means. For example, it is proposed to blend the hard ionomer resin having high rigidity with terpolymer-based soft ionomer resin (Japanese Patent Kokai Publications No. 3931/1993, Japanese Patent No. 2709950 and the like) or blend the hard ionomer resin with thermoplastic elastomer (Japanese Patent Kokai Publications Nos. 299052/1994, 327794/1994 and the like) to soften the cover.
However, when good shot feel and spin performance are accomplished in case of using the blend of the hard ionomer with the soft ionomer resin, rebound characteristics of the resulting golf ball are largely degraded. In addition, since the surface of the cover of the golf ball is easily abraded by grooves on a face surface of the golf club when hit by an iron club, it is problem that the surface of the golf ball becomes fluffy or begins to split finely, and scuff resistance is poor.
When the blend of the hard ionomer with the thermoplastic elastomer is used, rebound characteristics are excellent compared with the blend with the soft ionomer resin, but the compatibility between the both is poor, and scuff resistance when hit by an iron club is poor compared with the blend with the soft ionomer resin.
In order to solve the problems, it has been suggested to use thermosetting polyurethane composition (Japanese Patent Kokai publication No. 74726/1976, Japanese Patent No. 2662909, U.S. Pat. No. 4,123,061 and the like), or polyurethane-based thermoplastic elastomer (U.S. Pat. Nos. 3,395,109, 4,248,432, Japanese Patent Kokai publication Nos. 271538/1997, 1280401/1999, 1280402/1999 and the like) as a cover material of golf ball. When the thermosetting polyurethane composition is used for the cover material, the scuff resistance, which is problem from using the blend of the hard ionomer resin with the soft ionomer resin or thermoplastic elastomer, is excellent. However, since a process of coating a cover on a core is complicated, large-scale production is difficult, and the productivity is degraded.
When the polyurethane-based thermoplastic elastomer is used for the cover material, compared with the thermosetting polyurethane composition, the productivity is improved, but it is difficult to thin the cover layer. When polyurethane-based thermoplastic elastomer formed by using typical diisocyanates, such as 2,4-tolylene diisocyanate, 2,6-tolylene diisocyanate or a mixture thereof (TDI), 4,4′-diphenylmethane diisocyanate (MDI) and the like, is used for the cover material, its strength is high, and scuff resistance of the resulting golf ball is excellent, but there is problem that yellowing is easy to occur, and it is required to further use white paint. In order to solve the problem, it has been suggested to use polyurethane-based thermoplastic elastomer formed by using aliphatic diisocyanate for the cover material (Japanese Patent Kokai publication No. 271538/1997). However, there is problem that its strength is low, and scuff resistance of the resulting golf ball is degraded, compared with the polyurethane-based thermoplastic elastomer formed by using aromatic diisocyanate.